The present invention relates to a color cathode ray tube of the type which is used in a color television set for personal use and to a color display monitor for an information terminal; and, more particularly, the invention relates to a color cathode ray tube which has a faceplate panel of improved curved shape.
The glass envelope of a cathode ray tube generally comprises a panel portion having a curved faceplate, a neck portion with a reduced diameter, and an approximately funnel-shaped portion which connects the panel portion and the neck portion. The cathode ray tube further includes a phosphor screen formed over the inner surface of the faceplate, an electron gun installed inside of the neck portion, and a deflection yoke mounted on an outer periphery of the funnel portion. Here, the glass envelope of the cathode ray tube has a near vacuum in its interior, with atmospheric pressure being impressed on it, outer side at all times, so that the glass envelope is required to have a mechanical strength that is higher than a predetermined level. For this reason, various parts of the glass envelope are formed to a sufficient thicknesses to be able to provide the corresponding required mechanical strengths.
In a known cathode ray tube, the faceplate of the glass envelope normally has a construction in which the peripheral section of the faceplate is made thicker than the central section of the faceplate. FIG. 11 is a cross-sectional view showing one example of the constitution of the faceplate portion of a glass envelope of the type used in a known cathode ray tube.
In FIG. 11, numeral 31 indicates a faceplate, numeral 31(1) indicates an inner surface of the faceplate, numeral 31(2) indicates an outer surface of the faceplate, tpc indicates the thickness of a central section of the faceplate 31, tpa indicates the thickness of a peripheral section of the faceplate 31, Rpi indicates the radius of curvature of the inner surface 31(1) of the faceplate with a deflection center point O of the electron beam taken as its center, and Rpo indicates the radius of curvature of the outer surface 31(2) of the faceplate with the deflection center point O of the electron beam taken as its center.
As shown in FIG. 11, the faceplate 31 is constructed such that the thickness tpa of the peripheral section is greater than the thickness tpc of the central section so as to provide the required mechanical strength as described above. As a result, the radius of curvature Rpi of the inner surface 31(1) of the faceplate is smaller than the radius of curvature Rpo of the outer surface 31(2) of the faceplate, that is, tpc less than tpa and Rpi less than Rpo.
In the above-described known cathode ray tube, the thickness tpc of the central section of the faceplate 31 is thin and the thickness tpa of the peripheral section is thick so that when an image is displayed on the phosphor screen formed on the inner surface of the faceplate 31, light irradiated outwardly from the phosphor screen through the faceplate 31 becomes attenuated more at the peripheral section of the faceplate 31 having the large thickness tpa than at the central section of the faceplate 31 having the small thickness tpc. That is, if we let Tpc stand for the light transmittivity at the central section of the faceplate 31 and Tpa for a light transmittivity at the peripheral section, then Tpc greater than Tpa and the brightness of the displayed image is lower at the peripheral section of the faceplate 31 than at the central section of the faceplate 31, thus giving rise to a problem that the brightness of a displayed image cannot be ensured at a sufficient level at the peripheral section. The brightness at the peripheral section is further degraded by the fact that the weight of the phosphor is smaller at the peripheral section of the screen than at the central section of the screen.
To correct the brightness of the displayed image at the peripheral section of the faceplate 31 so that it will match the brightness of the displayed image at the central section, when the brightness of the displayed image at the peripheral section of the faceplate 31 is lowered compared with the brightness of the displayed image at the central section, the intensity of the electron beam projected onto the peripheral section of the phosphor screen needs to be stronger than the intensity of the electron beam projected onto the central section of the phosphor screen. Such a means for correcting the electron beam intensity, however, cannot be easily obtained.
To cope with such a problem, an applicant of the present invention filed an application (Japanese Laid-open Patent Publication 18547/1998), in which the curved shape of the faceplate of a cathode ray tube is formed such that the radius of curvature of the inner surface thereof is larger than the radius of curvature of the outer surface, and the thickness of the peripheral section is made thin compared with the thickness of the central section of the faceplate, so that the brightness of a displayed image at the peripheral section of the faceplate is increased, and the brightness of the displayed image at the peripheral section is approximated to the brightness of the displayed image at the central section.
However, in the above-mentioned cathode ray tube, since the radius of curvature of the inner surface of the faceplate is larger than that of the outer surface of the faceplate, the radius of curvature of a shadow mask, which is arranged to face the phosphor screen formed on the inner surface of the faceplate in an opposed manner and performs color selection, becomes large as a whole. In a cathode ray tube having such a constitution, since the molding retaining strength of the curved surface of the shadow mask becomes weak, the curved surface is easily deformed, so that the electron beams which pass through apertures formed in the shadow mask do not normally impinge on the phosphor screen (making it impossible to perform normal color selection) whereby there arises a new problem in that a deterioration of the color purity of the displayed image is induced.
Particularly, at the peripheral section of the screen along the minor axis direction, the shadow mask is liable to be easily influenced by vibrations or an impact at the time of dropping; and, hence, when the radius of curvature of the shadow mask is increased, the mechanical strength of the curved surface becomes weak, so that the curved surface is liable to be easily deformed in that area.
Further, the inside of the glass bulb is approximately in a vacuum state and atmospheric pressure is always applied to the outside of the glass bulb. Accordingly, in view of the geometric structure of the glass bulb for a cathode ray tube, a stress strain is liable to be concentrated on the peripheral section of the screen, along the minor axis direction in particularly; and, hence, when the glass thickness of the peripheral section along the minor axis direction becomes thin by increasing the radius of curvature of the inner surface of the faceplate, the mechanical strength of the glass bulb (panel portion) becomes weak in that area.
Further, at the peripheral section of the screen along the diagonal direction, the distance from the center of the screen becomes longest; and, hence, when the radius of curvature of the shadow mask becomes large, the molding retaining strength of the curved surface is decreased, so that the curved surfaces are liable to be easily deformed between the center and the peripheral section along the diagonal direction.
The present invention has been made to solve the foregoing problems, and it is an object of the present invention to provide a color cathode ray tube in which the brightness of a displayed image at a peripheral section of a faceplate will match the brightness of a displayed image at a central section of the faceplate using means having a simple constitution and, at the same time, can prevent the degradation of the color purity of the displayed image and the lowering of the mechanical strength of the glass bulb.
To achieve the above-mentioned object, in the cathode ray tube according to the present invention, the thickness of the glass and/or the radius of curvature of a faceplate panel are constituted as follows.
(1) Assuming a thickness of a screen effective area of a faceplate portion along the tube axis direction as tc at a central section thereof, as tx at a peripheral section along the major axis direction and as td at a peripheral section along the diagonal direction, the relationship tc greater than td greater than tx is established. Due to such a constitution, the brightness of a displayed image at the peripheral section is enhanced and, at the same time, the radius of curvature of the shadow mask between tire central section and the peripheral section along the diagonal direction can be made small, so that it becomes possible to prevent the lowering of the molding retaining strength of the curved surface.
(2) Assuming a thickness of a screen effective area of a faceplate portion along the tube axis direction as tc at a central section thereof and as tx at a peripheral section along the major axis direction, the relationship tc greater than tx is established, and assuming an equivalent radius of curvature of a screen effective area of an inner surface of the faceplate portion as Rix in the major axis direction and as Rid in the diagonal direction, the relationship Rix greater than Rid is established. Due to such a constitution, the brightness of a displayed image at the peripheral section is enhanced and, at the same time, the lowering of the molding retaining strength of the curved surface of the shadow mask between the central section and the peripheral section along the diagonal direction can be prevented.
(3) Assuming a thickness of a screen effective area of a faceplate portion along the tube axis direction as tc at a central section thereof, as tx at a peripheral section along the major axis direction and as ty at a peripheral section along the minor axis direction, the relationship tc greater than ty greater than tx is established. Due to such a constitution, the brightness of a displayed image at the peripheral section is enhanced and, at the same time, the lowering of the mechanical strength of an evacuated glass bulb (panel portion) at the peripheral section along the minor axis direction can be prevented. Further, the radius of curvature of the shadow mask at the peripheral section along the minor axis direction becomes small so that the lowering of the mechanical strength of the shadow mask can be prevented.
(4) Assuming a thickness of a screen effective area of a faceplate portion along the tube axis direction as tc at a central section thereof and as tx at a peripheral section along the major axis direction, the relationship tc greater than tx is established, and assuming an equivalent radius of curvature of the screen effective area of an inner surface of the faceplate portion as Rix in the major axis direction and as Riy in the minor axis direction, the relationship Rix greater than Riy is established. Due to such a constitution, the brightness of a displayed image at the peripheral section is enhanced and, at the same time, the lowering of the mechanical strength of an evacuated glass bulb (panel portion) and the shadow mask at the respective peripheral sections along the minor axis direction can be prevented.
(5) Assuming a thickness of a screen effective area of a faceplate portion along the tube axis direction as tc at a central section thereof, as ty at a peripheral section along the minor axis direction and as td at the peripheral section along the diagonal direction, the relationship tc greater than td greater than ty is established, and assuming an equivalent radius of curvature of the screen effective area of an inner surface of the faceplate portion as Riy in the minor axis direction and as Rid in the diagonal direction, the relationship Rid greater than Riy is established, and assuming an equivalent radius of curvature of a screen effective area of an outer surface of the faceplate portion as Roy in the minor axis direction and as Rod in the diagonal direction, the relationship Rod greater than Roy is established. Also, due to such a constitution, the brightness of a displayed image at the peripheral section is enhanced and, at the same time, the lowering of the mechanical strength of an evacuated glass bulb (panel portion) and the shadow mask at the respective peripheral sections along the minor axis direction can be prevented.
Further, in addition to the above-mentioned constitutions, when necessary, the hole transmittivity of a black matrix for forming phosphor dots or stripes on a phosphor screen is defined in a given range.
According to the above-mentioned constitutions of the present invention, the brightness of the displayed image at the peripheral section of the faceplate can be made to match the brightness of the displayed image at the central section so that when electron beams are irradiated to the phosphor screen on the inner surface of the faceplate, the brightness of the image displayed on the phosphor screen is prevented from being lowered at the peripheral section, whereby the uniformity of the brightness of the whole surface of the display screen can be maintained. Further, since it is no longer necessary to excessively increase the hole transmittivity of the black matrix at the periphery of the screen, a color cathode ray tube which does not give rise to a large deterioration of the resolution at the periphery of the screen and which exhibits a favorable color purity can be obtained.